(CBS News) Sen. Charles Grassley, R-Iowa, is investigating a mysterious rift between the Bureau of Alcohol, Tobacco and Firearms (ATF) and the U.S. Attorney's office that prosecutes ATF cases in Reno, Nevada. According to a letter from Grassley and a local report, the rift has meant that some potential firearms crimes have gone unprosecuted and unpunished.

The rift is referred to in an internal memo dated Sept. 29, 2011, in which the U.S. Attorney's office tells ATF, "At this time, we are not accepting any cases submitted by your office." The memo tells ATF that the U.S. Attorney will consider ATF cases "when your management addresses and resolves the issues at hand."

In a letter to Nevada U.S. Attorney Daniel Bogden, Grassley writes: "Refusing to even consider cases that might merit prosecution as a way to exert influence over a law enforcement agency appears to be an extremely broad application of your discretion."

As to what the issues are, neither side is saying publicly. The Reno Gazette-Journal reported that one victim whose case was dropped as a result of the squabble was "furious that prosecutors dropped the case."

Grassley has asked for a briefing of the issues by Wednesday.

Out of six agents assigned to ATF in Reno, four have transferred as a result of the conflict. That leaves just two agents left to investigate firearms crimes in the region.

Nobody answered the Reno U.S. Attorney's office today, and a call asking for comment from the main U.S. Attorney's office in Las Vegas was transferred to voicemail.

ATF and the Department of Justice (DOJ), which oversees the agency, are already under fire for the recent gunwalking scandals, and last week's Inspector General report that blasted DOJ and ATF officials for their handling of the case known as "Fast and Furious" in which ATF allowed thousands of weapons to fall into the hands of Mexican drug cartels. ATF agents claimed the Fast and Furious scheme grew, in part, out of the fact that the Arizona U.S. Attorney's office often wouldn't accept ATF's firearms cases, and allegedly set an unacceptably high bar for prosecution.